Emerging resistant Salmonella strain sparks concern

Aug 3, 2011 (CIDRAP News) – An international team of researchers reports that a strain of Salmonella that is resistant to important antibiotics has spread from Africa to Europe in recent years and has been spotted in a few samples of imported spices in the United States.

The new strain, a variant of Samonella enterica serotype Kentucky, caused illness in 489 people in three European countries between 2002 and 2008, many of whom had traveled to North Africa or the Middle East, according to a report published today in Clinical Infectious Diseases. The pathogen was detected increasingly often over that period, it says.

The new strain is resistant to several antimicrobials and shows high-level resistance to ciprofloxacin, a member of the fluoroquinolone class, which is important for treating severe Salmonella infections, the report says.

The study was triggered by the detection of ciprofloxacin-resistant Salmonella Kentucky infections in 17 French travelers who had visited eastern or northeastern Africa from 2002 to 2005. The researchers gathered Salmonella surveillance data from France, England and Wales, Denmark, and the United States for the period 2002 through 2008 and ran molecular analyses on isolates from these countries and from possible source countries.

They found that 200 ciprofloxacin-resistant Salmonella Kentucky isolates were reported in France, 244 in England and Wales, and 45 in Denmark, for a total of 489, the report says. In the United States, 679 Salmonella Kentucky isolates were reported to the US Centers for Disease Control and Prevention (CDC) during the study period, but just 23 of these were submitted to the National Antimicrobial Resistance Monitoring System, and none of these were ciprofloxacin-resistant.

However, using the US PulseNet database of pulsed-field gel electrophoresis (PFGE, or DNA fingerprint) data for Salmonella Kentucky isolates, the researchers found five isolates similar to the strain they were looking for, isolated from spices imported from North Africa during 2002 to 2009. Three of these were ciprofloxacin-resistant.

The team found that the resistant strain grew more common over the study period, with just 3 isolates identified in 2002 and 174 in 2008. Travel information was available for 307 of the 489 patients, and it showed that 89% of them had traveled to Africa or Asia in the 15 days before their illness. Morocco and Egypt were the leading destinations by far.

Genetic studies led to the conclusion that all the human isolates belonged to a single clone characterized by certain multi-locus sequencing and PFGE patterns and the presence of a particular "genomic island." They dubbed it ST198-X1 (ciprofloxacin-resistant).

From other evidence, the authors conclude that the ancestor of this variant first appeared in the 1990s in Egypt, where it infected humans and acquired resistance determinants. They report that the strain has been found in poultry in Ethiopia, Morocco, and Nigeria, and they speculate that the widespread use of fluoroquinolones in chicken and turkey production in Nigeria and Morocco might have contributed to its spread. Its importance as a cause of illness in those countries is unknown.

Many of the human isolates were found to be resistant to more than just fluoroquinolones, the report says. Most notably, half of them had a resistance pattern that included amoxicillin, streptomycin, spectinomycin, gentamicin, sulfamethoxazole, and tetracycline.

The authors say that the 489 cases they identified probably represent an underestimate, because the surveillance systems of the respective countries do not cover all areas within their borders.

They also suggest that the resistant strain has probably reached North America, on the basis of the findings in imported spices in the United States and a report from Canada in 2006. Salmonella Kentucky has been the most common Salmonella serotype in US poultry, though it has caused few illness cases, the report notes.

"The current emergence of ST198-X1 . . . highlights the need to set up a global integrated national surveillance system for humans and all sectors of the food production chain, focused on the main foodborne pathogens in Africa," the authors conclude.

In an accompanying commentary, foodborne disease expert Craig W. Hedberg, PhD, of the University of Minnesota's School of Public Health, writes that the study demonstrates "the power and importance of linking public health surveillance systems on an international level."

"The emergence of multidrug-resistant S. Kentucky was identified before the clone spread to poultry flocks in Europe or the United States," Hedberg says. "Producers and regulators can now look for these strains and rapidly respond to their presence."

He says the discovery of the strain on samples of imported spices suggests that US agriculture systems are vulnerable, because contaminated food ingredients could lead to contamination of animal feed as well as human food. Further, he writes that the spread of the resistant variant "could be a major public health threat, similar to the earlier emergence and global dissemination of Salmonella Typhimurium DT104."

Hedberg also comments that "visionary and strong political leadership" is needed to maintain public health surveillance in the current era of budget-cutting.